Image sensors, such as complementary metal oxide semiconductors (CMOS), include a focal plane array of pixels. Each pixel in a CMOS image sensor, for instance, includes a photosensor, such as, a photogate, a photoconductor or a photodiode overlying a substrate. A readout circuit is provided for each pixel and includes at least a source follower transistor and a row select transistor for coupling the source follower transistor to a column output line. A pixel also typically has a floating diffusion region, connected to a gate of the source follower transistor.
A pixel's photosensor collects photons (incoming light) and converts the photons into an electrical charge. The image sensor may also include a transfer transistor for transferring the electrical charge generated by the photosensor to the floating diffusion region and a reset transistor for resetting the floating diffusion region to a predetermined charge level prior to charge transference. The source follower transistor converts the charge stored at the floating diffusion region into two electrical output signals Vrst, representing the reset state of the floating diffusion region, and Vsig, representing the electrical charge generated by the photosensor. The output signals Vrst, Vsig are subtracted and typically amplified to a signal level for further processing by an analog-to-digital converter (ADC) which produces a digital value used to produce a digital image.
The quality of the final digital image depends on many factors, one of which is the dynamic range of the'image sensor. The dynamic range of an image sensor is defined by the largest possible signal in relation to the smallest possible signal the image sensor can generate. The largest possible signal being proportional to the full well capacity of a pixel, i.e., the total amount of photons that the pixel can convert to electrons. The smallest possible signal is the noise level when the image sensor is not exposed to any light, that is generally referred to as the noise floor. Therefore, to capture the full dynamic range of a scene, that is capturing both shadow and highlight detail, an image sensor should have a wide dynamic range.
Applications, for example digital cameras, with imager sensors generally require a high dynamic range of at least 120 dB to capture the scene illuminations ranges (e.g., from 10−1 lux for dark scenes to 105 lux for scene having bright sunlight or direct headlights). Typical CMOS image sensors are unable to attain this high dynamic range due to the limitations of the full well and noise floor. Typical CMOS image sensors have a dynamic range of around 70 dB. Consequently, a typical image sensor will generally be unable to capture all details from deep shadows to bright highlights of a scene.
Image sensors can also include built in skip modes in which pixels in certain columns and rows of an array are not readout. Skip modes are used to readout only a portion of an image array which decreases the readout time but also reduces output resolution in a given field of view. Typical column skip mode operations, however, will leave a substantial part of the column readout circuit unused, e.g., half and three-fourths for “2×” and “4×” skip modes, respectively. It would be advantageous to utilize the unused channels of the readout circuit during skip mode readout operation of an imager pixel array.